1. Field of the Invention
The present invention relates to the field of thermal effects on materials, and in particular, the analysis of the thermal history of a material for monitoring the structural integrity of material components exposed to temperature stressors.
2. Description of the Related Art
Material components exposed to high temperatures (above 400 kelvin) for various periods of time suffer degradation in their mechanical properties. Such degradation may ultimately lead to failure of the component, especially where such materials are exposed to repeated temperature cycles, extreme temperatures, rapid heating and cooling, or accompanied by other stressors, such as pressure and oxidation. Material components, such as those on a jet aircraft, for example, are routinely exposed to a combination of these stressors. Many such components are primarily affected by thermal stressors, which contribute to microstructural changes, creep, thermal stresses, thermal fatigue, and thermal shock, as well as oxidation-induced degradation of the material. Such components need to be replaced after, or preferably before, significant degradation occurs. Failure of components on aircraft, as a result of thermally induced degradation, can cause catastrophic damage in both economic and human terms.
In the case of jet aircraft, regular maintenance and replacement of critical components is therefore essential to prevent catastrophic damage. Aircraft components particularly subject to thermal stressors are typically replaced after a predetermined number of flight hours or after fractures or other defects are detected. These solutions, while generally effective, may result in either the premature replacement of a costly component or the failure to replace the component when necessary, resulting in a catastrophic failure.
If it were possible to accurately determine the remaining useful lifetime of an aircraft component, a mechanic or technician could replace the component before catastrophic failure and without prematurely replacing a component having a significant useful lifetime remaining. This would further reduce operating costs by facilitating the smooth operation of aircraft, as the aircraft components can be ordered as needed, thereby avoiding downtime and production or delivery delays while waiting for critical components. This would also reduce the cost of storing components that may not be needed for significant periods of time.
In an effort to reduce these costs, many attempts have been made to determine, with increasing accuracy, the useful lifetimes of aircraft components. These attempts generally involve an effort to determine the temperature that such components have been exposed to. Temperature indicating paints, for example, have been used to determine whether a threshold temperature has been reached. These paints, which are applied to a component, change color or burn away after being exposed to a specific temperature. While such techniques are useful for indicating that a component was exposed to a specific temperature, they do not reveal by how much the specific temperature was exceeded or for how long the component was exposed to any given temperature—both of which are important factors in determining the expected remaining useful lifetime of a component. Indeed, in some cases, the time of exposure at a specific temperature can be as important or an even more important factor in determining the useful lifetime of a component as the specific temperature of exposure itself.
Others have attempted to obtain more information about the time of exposure at a given temperature. For example, U.S. Pat. No. 5,975,758 issued Nov. 2, 1999 to Yokota et al., provides a sensor having electrodes for measuring the change in resistance of a material. This technique is intended to provide information about the time of exposure at a given temperature. Such procedures, however, cannot distinguish between different stressors having vastly different effects on the useful lifetime of components. In other words, the same change in resistance may be caused by either the brief exposure at a specific temperature or a longer exposure at a different temperature, though the useful lifetime typically depends on which specific stressor occurred. In addition, the change in resistance is rapid above a threshold temperature for a given material, thereby yielding less useful information about the most important temperature stressors. The device also utilizes lead wires, which limit the maximum temperature of exposure. Moreover, the device fails to distinguish the sequence of stressors—which can significantly impact the useful lifetime of a component. Finally, for those materials specifically designed to operate at high temperatures, significant degradation of components may occur at intermediate temperatures, such as during cool-down. These devices provide little, if any, such information about intermediate temperature exposure once the component has been exposed to higher operating temperatures.
Though a means to continuously or periodically monitor the temperature of a component, using a thermocouple, for example, would provide more valuable information, such methods may not be feasible at very high temperatures. In addition, such methods require a great deal of memory for storing a large amount of data, particularly where multiple sensors are desired, and may not be practical or possible on certain surfaces given, for example, the physical requirements for storing the data—which can themselves degrade under high operating temperatures.
Accordingly, there is a need for an improved method and sensor for accurately obtaining the thermal history, in both normal and extreme operating environments, of a variety of components simultaneously, and without generating an excessive amount of data. In particular, there is a need for determining how long and at what temperatures a material has been exposed to, and in some cases, for determining the order such temperature stressors have been applied.